In an internal combustion engine, each main-bearing assembly typically comprises a pair of half bearings retaining a cylindrical journal of a crankshaft. Each half bearing comprises a semi-cylindrical bearing shell. The bearing shells typically have a layered construction in order to provide the required mechanical performance. A strong backing material provides structural strength, and is coated with one or more layers having preferred tribological properties to provide a bearing surface that faces and supports the cooperating crankshaft journal. The backing is usually of steel, of thickness 1 mm or more, and is coated with or bonded to a lining layer (which may comprise more than one individual layer), and an overlay which faces the crankshaft journal.
The function of the overlay is to provide a relatively soft, conformable layer that can accommodate any small misalignments between the harder steel crankshaft journal and the bearing shells, and receive and embed dirt particles that may circulate in the oil supply and enter the bearing, so as to prevent damage to or scoring of the journal. These functions of the overlay are respectively termed conformability and embeddability.
These functions require the overlay to be relatively soft and deformable, but at the same time sufficiently robust to provide a bearing surface having a long lifetime, without cracking due to overloading or fatigue and without unacceptable wear.
The overlay is made from an overlay material, which may be a metal layer, comprising, for example, lead, tin, bismuth, silver, copper alloys or aluminium alloys. The overlay may be applied to a substrate by galvanic processes, sputtering or electroplating. Alternatively, the overlay material may be a non-metal material comprising an artificial resin base, or matrix, and an additive for enhancing the load carrying capacity and/or wear resistance of the sliding component.
It is generally known that wear of the overlay material can lead to exposure of the lining layer to which the overlay material is applied. This can lead to failure of the sliding component due to seizure.
Trends in modern high speed diesel engines towards higher power outputs with downsized engine systems has further led to increased peak cylinder pressures, higher torques and higher temperatures. These factors mean that bearing materials, the upper con-rod and lower main bearing in particular, are required to perform under increased loads with a reduction in oil film thickness and viscosity.
A particular challenge to the performance of overlays has arisen in recent years from the fuel-saving configuration of modern vehicle engines to “stop-start” operation. Although engine bearings are conventionally hydrodynamically lubricated during use, little or no lubrication may be provided to the bearings when an engine starts. Accordingly, stop-start operation of an engine places increased demands upon the performance of the bearings, and in particular the performance of overlays.
Research in the field of sliding components has resulted in the use of a wide range of compositions of overlay materials, some incorporating relatively harder particles, particularly where the bulk of the overlay material is an artificial resin base or matrix, with the intention of providing the sliding component with enhanced wear resistance. There has been some interest in the use of iron oxides in overlay materials, for example in the following prior art.
Published international patent application number WO2010076306 in the name of Federal-Mogul Wiesbaden GmbH describes a sliding element having a substrate and a sliding layer material applied to the substrate. The sliding layer material comprises at least one cross-linkable binder or at least one high-melting thermoplastic material or is of a material that contains a matrix of at least one high-melting thermoplast material or at least one “Duroplast” material. The sliding layer material contains Fe2O3 with a preferred volume percentage fraction of 0.1 to 15 vol. %.
Federal-Mogul Wiesbaden GmbH sells crankshaft bearings and conrods under the trade name “IROX”® which comprise a substrate and an overlay consisting of polyamide imide (PAI) and Fe2O3 particles.
Although attempts have thus been made to improve the performance of overlays for sliding components, there is still a significant demand for improved overlay performance.